The present invention relates, in general, to current sensing devices, and more particularly, to current sensing devices used in continuous current mode, up/down DC-DC converters.
DC to DC converters are essential to providing accurate power delivery to sensitive electronic devices. In particular, devices which are designed to operate from a battery. Virtually all mobile devices such as cellular telephones, pagers and laptop computers, for example, require a regulated power supply, which is usually generated from a battery source, such as a NiCad (nickel-cadmium) or Lithium Ion battery source, for example. The amount of time that a battery is able to supply power to a particular device is directly proportional to the charge storage capacity of the battery and the amount of current required by the device. An imperative feature of any DC to DC converter operating from a battery, therefore, is the capability to not only monitor the amount of current delivered to the device by the converter, but the ability of the converter to detect an over-current or short circuit condition. An over-current condition is an anomaly, usually caused by a short circuit condition at the output of the converter, requiring the converter to detect the anomaly and subsequently shut down the converter in order to preserve battery life. High current peaks can damage batteries such as a lithium ion battery, therefore, current peaks must be limited, not only during normal operation, but during start up as well. DC-DC converters operating from battery power supplies must also have the ability to either provide up conversion or down conversion, depending on the voltage of the battery. Up conversion, for example, is needed for a weak battery which is supplying only 2.7 volts, for example, to a DC-DC converter, which is supplying power to a 5 volt load. Down conversion is needed for a fresh battery delivering 6 volts, for example, to the DC-DC converter supplying power to the 5 volt load.
Prior art current detectors for up/down DC-DC converters generally are positioned within the converter circuit, such that only high side current detection is possible. High side current detection occurs only for the up conversion state of the DC-DC converter, whereas, current detection is not performed for the low side, or down conversion, mode. A need exists, therefore, to provide current detection and over-current protection in an up/down DC-DC converter, for both high and low side conversion. Over-current limitation must also be provided during the start up phase of the converter. Additionally, average current detection is preferred over peak current detection, to prevent converter shut down during momentary output current surges.